U.S. patent number 6,613,123 [Application Number 09/863,763] was granted by the patent office on 2003-09-02 for variable melting point solders and brazes.
Invention is credited to Stephen F. Corbin, Douglas J. McIsaac, Xin Qiao.
United States Patent |
6,613,123 |
Corbin , et al. |
September 2, 2003 |
Variable melting point solders and brazes
Abstract
Variable melting point solders and brazes having compositions
comprising a metal or metal alloy powder having a low melting point
with a metal powder having a higher melting point. Upon heating,
in-situ alloying occurs between the low and high melting point
powders such that solidification occurs at the solder or braze
temperature thus creating a new, higher solidus (or melting)
temperature with little or no intermetallic formation. A solder
comprising Sn powder mixed with a Sn--Bi eutectic powder having a
composition of 63 wt % Sn:57 wt % Bi such that the bulk composition
of the mixture is 3 wt % Bi has an initial melting point of
140.degree. C. and a re-melt temperature of 220.degree. C. after
heating due to in-situ alloying. A composition of Pb powder mixed
with a Pb--Sn eutectic powder having a composition of 62 wt % Sn:58
wt % Pb such that the bulk composition of the mixture is 15 wt % Sn
has an initial melting point of 183.degree. C. and a re-melt
temperature of 250.degree. C.
Inventors: |
Corbin; Stephen F. (Waterloo,
Ontario, CA), McIsaac; Douglas J. (Waterloo, Ontario,
CA), Qiao; Xin (Boise, ID) |
Family
ID: |
22767956 |
Appl.
No.: |
09/863,763 |
Filed: |
May 24, 2001 |
Current U.S.
Class: |
75/255; 148/24;
252/513; 252/514; 252/520.1; 75/245; 252/518.1 |
Current CPC
Class: |
B23K
35/302 (20130101); B22F 1/0003 (20130101); B23K
35/262 (20130101); B23K 35/268 (20130101); B23K
35/264 (20130101); B23K 35/0244 (20130101); H05K
3/3485 (20200801); B23K 35/28 (20130101); B23K
35/3033 (20130101); B23K 35/288 (20130101); B23K
35/0227 (20130101); H05K 2201/0272 (20130101); B23K
35/025 (20130101); H05K 3/3463 (20130101); B23K
35/0233 (20130101) |
Current International
Class: |
B22F
1/00 (20060101); B23K 35/26 (20060101); B23K
35/02 (20060101); B23K 35/30 (20060101); B23K
35/28 (20060101); H05K 3/34 (20060101); B22F
001/00 (); B22F 003/00 () |
Field of
Search: |
;75/255,245
;252/513,518.1,520.1,514 ;148/24 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
3424635 |
|
Jan 1986 |
|
DE |
|
0867255 |
|
Dec 1997 |
|
EP |
|
9743081 |
|
Nov 1997 |
|
WO |
|
Primary Examiner: Mai; Ngoclan
Attorney, Agent or Firm: Schumacher; Lynn C.
Parent Case Text
CROSS REFERENCE TO RELATED U.S. APPLICATION
This application relates to U.S. Provisional patent application,
Serial No. 60/206,786, filed on May 24, 2000, entitled VARIABLE
MELTING POINT SOLDERS AND BRAZES.
Claims
Therefore what is claimed is:
1. A composition for soldering, comprising: a powder mixture
including a first constituent having a first melting point, said
first constituent having a preselected first mean particle size,
said first constituent including a first metal or metal alloy
powder; and a second constituent including a second metal powder
having a second melting point and a preselected second mean
particle size, the first melting point being lower than the second
melting point, said first and second mean particle sizes being
selected so that upon heating to a solder temperature, such
temperature being above the first melting point and below the
second melting point, in-situ alloying occurs between melted first
metal or metal alloy powder and the second metal powder in such a
way that solidification occurs at the solder temperature with
substantially no intermetallic phase formation.
2. The composition according to claim 1 wherein said first mean
particle size and said second mean particle size are selected to
give a maximum rate of initial melting with minimal in-situ
alloying prior to melting, and a maximum rate of in-situ alloying
after melting, resulting in solidification at the solder
temperature.
3. The composition according to claim 1 wherein said first mean
particle size and said second mean particle size are selected to
give a desired solidification rate of a low melting point liquid
produced by melting of the first constituent.
4. The composition according to claim 3 wherein said first mean
particle size and said second mean particle size are in a range
from about 500 to 600 mesh to promote rapid solidification of said
low melting point liquid.
5. The composition according to claim 3 wherein said first mean
particle size and said second mean particle size are in a range
from about 200 to about 325 mesh to delay solidification of said
low melting point liquid.
6. The composition according to claim 3 wherein said first mean
particle size is in a first range and said second mean particle
size is in a second range to give intermediate rates of
solidification of said low melting point liquid.
7. The composition according to claim 1 wherein said mixture is
compacted into wires or sheets.
8. The composition according to claim 1 wherein said mixture is
tape cast into preselected shapes.
9. The composition according to claim 1 wherein said mixture is
mixed with a flux to form a paste.
10. The composition according to claim 1 wherein said first mean
particle size and said second mean particle size are in a range
from about 100 mesh to about 600 mesh.
11. A composition for soldering, comprising: a Pb--Sn eutectic
powder mixed with a substantially pure Pb powder, wherein a bulk
composition of Sn in the mixture is in a range from about 5 wt % to
about 21 wt %, said Pb powder including particles having a first
mean particle size and said Pb--Sn eutectic powder including
particles having a second mean particle size, said Pb powder and
Pb--Sn eutectic powder having mean particle sizes selected so that
upon heating to a solder temperature, such solder temperature being
above the Pb--Sn eutectic melting point and below the melting point
of said substantially pure Pb powder, in-situ alloying occurs
between melted Pb--Sn eutectic powder particles and Pb powder
particles in such a way that solidification occurs at the solder
temperature with substantially no intermetallic phase
formation.
12. The composition according to claim 11 wherein said bulk
composition of Sn in the mixture is about 15 wt % Sn.
13. The composition according to claim 12 wherein said mixture has
an initial melting temperature of about 183.degree. C. and a remelt
temperature of about 250.degree. C.
14. The composition according to claim 11 wherein said mixture is
mixed with a flux to form a paste.
15. The composition according to claim 14 wherein said paste
comprises about 90 wt % of the powder mixture and about 10 wt % of
said flux.
16. The composition according to claim 15 wherein said flux is an
activated rosin solution.
17. The composition according to claim 12 wherein both the Pb
powder and eutectic Pb--Sn powder have powder sizes in a range from
about 200 mesh to about 325 mesh.
18. The composition according to claim 11 wherein both the Pb
powder and eutectic Pb--Sn powder have powder sizes in a range from
about 500 mesh to about 600 mesh.
19. A composition for brazing, comprising: a powder mixture
including a first constituent having a first melting point, said
first constituent having a preselected first mean particle size,
said first constituent including a first metal or metal alloy
powder; and a second constituent including a second metal powder
having a second melting point and a preselected second mean
particle size, the first melting point being lower than the second
melting point, said first and second mean particle sizes being
selected so that upon heating to a braze temperature, such
temperature being above the first melting point and below the
second melting point, in-situ alloying occurs between melted first
metal or metal alloy powder and said second metal powder in such a
way that solidification occurs at the braze temperature with
substantially no intermetallic phase formation.
20. The composition according to claim 19 wherein said first mean
particle size and said second mean particle size are selected to
give a maximum rate of initial melting with minimal in-situ
alloying prior to melting, and a maximum rate of in-situ alloying
after melting, resulting in solidification at the braze
temperature.
21. The composition according to claim 19 wherein said first mean
particle size and said second mean particle size are selected to
give a desired solidification rate of a low melting point liquid
produced by melting of the first constituent.
22. The composition according to claim 21 wherein said first mean
particle size and said second mean particle size are in a range
from about 500 to 600 mesh to promote rapid solidification of said
low melting point liquid.
23. The composition according to claim 21 wherein said first mean
particle size and said second mean particle size are in a range
from about 200 to about 325 mesh to delay solidification of said
low melting point liquid.
24. The composition according to claim 21 wherein said first mean
particle size is in a first range and said second mean particle
size is in a second range to give intermediate rates of
solidification of said low melting point liquid.
25. The composition according to claim 19 wherein said powder
mixture is compacted into wires or sheets.
26. The composition according to claim 19 wherein said powder
mixture is tape cast into preselected shapes.
27. The composition according to claim 19 wherein said powder
mixture is mixed with a flux to form a paste.
28. A composition for soldering according to claim 1 wherein the
first constituent is substantially pure tin powder and the second
constituent is Sn--Bi eutectic powder having a composition of about
43 wt % Sn:57 wt % Bi and wherein a bulk composition of Bi in the
mixture is in a range from about 2 wt % to about 19 wt %.
29. The composition according to claim 28 wherein said bulk
composition of Bi in said mixture is about 3 wt % Bi.
30. The composition according to claim 29 wherein said mixture has
an initial melting temperature of about 140.degree. C. and a remelt
temperature of about 220.degree. C.
31. The composition according to claim 29 wherein said mixture is
mixed with a flux to form a paste.
32. The composition according to claim 31 wherein said paste
comprises about 90 wt % of the mixture and about 10 wt % of said
flux.
33. The composition according to claim 32 wherein said flux is an
activated rosin solution.
34. The composition according to claim 28 wherein both the Sn
powder and Sn--Bi eutectic powder have powder sizes in a range from
about 200 mesh to about 325 mesh.
35. The composition according to claim 28 wherein both the Sn
powder and Sn--Bi eutectic powder have powder sizes in a range from
about 500 mesh to about 600 mesh.
36. A composition for soldering according to claim 1 wherein the
first constituent is substantially pure Sn powder and the second
constituent is substantially pure antimony (Sb) powder, and wherein
a bulk composition of Sn in the mixture includes 10 wt % Sn.
37. The composition according to claim 36 wherein said mixture has
an initial melting temperature of about 232.degree. C. and a remelt
temperature in a range from about 240 to about 245.degree. C.
38. The composition according to claim 36 wherein said mixture is
mixed with a flux to form a paste.
39. The composition according to claim 38 wherein said paste
comprises about 90 wt % of the mixture and about 10 wt % of said
flux.
40. The composition according to claim 39 wherein said flux is an
activated rosin solution.
41. The composition according to claim 36 wherein the Sn powder has
a powder size is in a range from about 200 mesh to about 325 mesh
and the Sb powder has a powder size in a range from about 200 mesh
to about 600 mesh.
42. The composition according to claim 41 wherein the Sb powder
size is in a range from about 500 mesh to about 600 mesh.
43. A composition for soldering according to claim 1 wherein the
first constituent is substantially pure Bi powder and the second
constituent is substantially pure antimony (Sb) powder, and wherein
a bulk composition of antimony in the mixture is about 10 wt %
Sb.
44. The composition according to claim 43 wherein said mixture has
an initial melting temperature of about 272.degree. C. and a remelt
temperature of about 285.degree. C.
45. The composition according to claim 43 wherein said mixture is
mixed with a flux to form a paste.
46. The composition according to claim 45 wherein said paste
comprises about 90 wt % of the mixture and about 10 wt % of said
flux.
47. The composition according to claim 46 wherein said flux is an
activated rosin solution.
48. The composition according to claim 43 wherein the Bi powder has
a powder size in a range from about 200 mesh to about 325 mesh and
the Sb powder has a powder size in a range from about 200 mesh to
about 600 mesh.
49. The composition according to claim 48 wherein the Sb powder
size is in a range from about 500 mesh to about 600 mesh.
50. A composition for brazing according to claim 19 wherein the
second constituent is substantially pure Ni and the first
constituent is substantially pure Cu powder such that a bulk
composition of Ni in the mixture is in a range from about 15 wt %
to about 85 wt % Ni.
51. The composition according to claim 50 wherein said bulk
composition of Ni in the mixture is about 25 wt % Ni.
52. The composition according to claim 50 wherein the Cu and Ni
powder size is in a range from about 200 mesh to about 325
mesh.
53. The composition according to claim 50 wherein the Cu powder
size is in a range from about 200 mesh to about 325 mesh, and
wherein the Ni powder size is in a range from about 48 mesh to
about 200 mesh.
54. The composition according to claim 51 wherein said composition
has an initial melting temperature of about 1085.degree. C. and a
remelt temperature of about 1175.degree. C.
55. A composition for brazing according to claim 19 wherein the
first constituent is Cu--Ag eutectic alloy powder and the second
constituent is substantially pure Cu powder such that a bulk
composition of Ag in the mixture is between about 5 to about 8 wt %
Ag.
56. A composition for brazing according to claim 19 wherein the
first constituent is Al--Zn eutectic alloy powder and the second
constituent is substantially pure Al powder such that a bulk
composition of Zn in the mixture is between about 10 to 20 wt %
Zn.
57. A composition for brazing according to claim 19 wherein the
first constituent is Cu--P eutectic alloy powder and the second
constituent is substantially pure Cu powder such that a bulk
composition of P in the mixture is between about 1 to 2 wt % P.
58. A composition for brazing according to claim 19 wherein the
first constituent is Cu--Mn alloy powder and the second constituent
is substantially pure Ni powder such that the composition of the
braze is about 80% Ni, 14% Cu and 6% Mn.
59. A composition for brazing according to claim 19 wherein the
first constituent is Cu--Sn alloy powder and the second constituent
is substantially pure Ni powder such that the composition of the
braze is 80% Ni, 10% Cu, and 10% Sn.
Description
FIELD OF THE INVENTION
This invention relates in general to solder and brazing materials
and more particularly to variable melting point solders and
brazes.
BACKGROUND OF THE INVENTION
Solder pastes are used to form joints between components and
printed circuit boards (PCB) in the microelectronics industry. The
solder provides both a physical and electrical bond. A conventional
solder paste comprises a powder, made from a solder alloy, which is
suspended in a liquid vehicle that contains a flux. The paste is
dispensed onto a PCB, for example, by a screen-printing process.
Components to be attached to the PCB are placed into the paste
where they are held in place by the high viscosity of the paste.
This assembly is heated such that the solder powder melts and
coalesces to form a dense liquid whereupon it spreads over the
component and PCB surface thus forming a metallurgical joint. Upon
cooling the liquid solder solidifies and a solid joint is formed
with the required mechanical, electrical and thermal properties.
Common solder powders consist of alloyed metals such as lead (Pb)
and tin (Sn) having compositions which give low melting points
(i.e. eutectic compositions), which is advantageous from a
processing point of view. A key characteristic of these solders is
that they have a single reproducible melting point.
There are situations during the fabrication of microelectronics
where stepwise soldering would be advantageous. In these cases,
some components are soldered to a PCB, the assembly taken to
another operation and then a third operation where additional
components are soldered to a PCB. In yet another manufacturing
method, components are soldered to one side of a PCB and other
components are later soldered to the opposite side of the PCB.
In these cases the solder used in the first step must have a
melting temperature higher than the solder used in the second step
to avoid remelting of the solder from the first step. The use of
more than one solder complicates, adds expense and can adversely
affect the performance of the joints in an application.
U.S. Pat. Nos. 5,540,379, 5,573,602, and 5,803,340 are directed to
a solder paste consisting of a low melting point and a high melting
point metal (or alloy). When the paste is heated to the melting
point of the low melting point powder the latter melts and while
this phase is still liquid the paste is heated a second time to a
temperature high enough to melt the rest of the mixture followed by
cooling. These pastes are formulated with a composition such that
most or all of the metal is liquid at the highest solder
temperature.
These pastes use complex solder compositions with three or more
alloying elements, contain lead (Pb) and high contents of low
melting point elements such as bismuth (Bi) and indium (In). Since
the solders are liquid at the soldering temperature and have a
complex composition, the formation of low temperature phases which
can segregate to particle or grain boundaries during freezing can
be formed. These phases are detrimental to physical and mechanical
properties of the resulting solder joint.
U.S. Pat. No. 5,229,070 teaches a solder produced by mixing a high
tin (Sn) content alloy powder with a Sn powder substantially
alloyed with either In or Bi. The Sn powder alloyed with Bi or In
melts at a lower temperature than the high Sn content powder. The
process consists of a single soldering step in which the paste is
heated to a high enough temperature to melt all the components in
the mixture. The primary purpose of this solder paste is to improve
wetting and reduce the time of soldering. The solder is not
designed for stepwise soldering and solidification at the soldering
temperature does not occur.
U.S. Pat. No. 4,834,794 teaches a solder paste using a low and high
melting point solder along with a reactive powder to form the
paste. The objective of this patent is to provide a solder that has
a low melting point and a higher remelting point. This is achieved
by a reaction between the powders to form intermetallic compounds.
A drawback to this type of solder relates to the fact that
intermetallic compounds tend to be brittle and detrimental to the
mechanical properties of the solder joint.
Thus the development of a solder which can melt the first time at a
low temperature but not remelt at that temperature in subsequent
solder operations (i.e. acquire a higher remelt temperature) would
be of significant value in microelectronics where stepwise
soldering is required or would be advantageous.
In the manufacture of light bulbs, solder joints are formed on an
automated production line. It is advantageous to have a solder
which melts at a low temperature so the cost of the soldering
process can be minimized. However, when the light bulb is put into
an application the melting point of the solder must be high enough
that it does not re-melt during use. Therefore when using
conventional solders, which have a single reproducible melting
point, the soldering temperature cannot be lower than the service
temperature. Thus the development of a solder which can melt the
first time at a low temperature to reduce manufacturing costs but
obtain a higher re-melt temperature during soldering such that it
does not melt at the service temperature, would be
advantageous.
In many automated furnace brazing applications it is desirous to
have as low a brazing temperature as possible to minimize any
detrimental microstructural changes that can occur in the base
materials to be brazed. However, the service requirements again
mean that the brazing temperature can not be lower than that
experienced in the application. Thus the development of a braze
material which can melt the first time at a low temperature to
reduce damage to the base materials but obtain a higher re-melt
temperature during brazing such that it does not melt at the
service temperature, would be advantageous.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide variable
melting point solders and brazes.
The present invention provides a composition for soldering,
comprising: a powder mixture including a first constituent having a
first melting point, said first constituent having a preselected
first mean particle size, said first constituent including a first
metal or metal alloy powder; and a second constituent including a
second metal powder having a second melting point and a preselected
second mean particle size, the first melting point being lower than
the second melting point, said first and second mean particle sizes
being selected so that upon heating to a solder temperature, such
temperature being above the first melting point and below the
second melting point, in-situ alloying occurs between melted first
metal or metal alloy powder and the second metal powder in such a
way that solidification occurs at the solder temperature with
substantially no intermetallic phase formation.
The first mean particle size and said second mean particle size are
selected to give a maximum rate of initial melting with minimal
in-situ alloying prior to melting, and a maximum rate of in-situ
alloying and solidification at the solder or brazing
temperature.
The present invention also provides a composition for soldering,
comprising: a Pb--Sn eutectic powder mixed with a substantially
pure Pb powder, wherein a bulk composition of Sn in the mixture is
in a range from about 5 wt % to about 21 wt %, said Pb powder
including particles having a first mean particle size and said
Pb--Sn eutectic powder including particles having a second mean
particle size, said Pb powder and Pb--Sn eutectic powder having
mean particle sizes selected so that upon heating to a solder
temperature, such solder temperature being above the Pb--Sn
eutectic melting point and below the melting point of said
substantially pure Pb powder, in-situ alloying occurs between
melted Pb--Sn eutectic powder particles and Pb powder particles in
such a way that solidification occurs at the solder temperature
with substantially no intermetallic phase formation.
In this aspect of the invention the bulk composition of Sn in the
mixture is about 15 wt % Sn and the mixture has an initial melting
temperature of about 183.degree. C. and a remelt temperature of
about 250.degree. C.
The present invention provides a composition for soldering,
comprising: a Sn--Bi eutectic powder having a composition of about
63 wt % Sn:57 wt % Bi mixed with a substantially pure tin (Sn)
powder, and wherein a bulk composition of Bi in the mixture is in a
range from about 2 wt % to about 19 wt %.
In this aspect of the invention the bulk composition of Bi in said
mixture is about 3 wt % Bi.
The present invention provides a composition for soldering,
comprising: substantially pure Sn powder mixed with substantially
pure antimony (Sb) powder, and wherein a bulk composition of Sn in
the mixture includes 10 wt % Sn.
In this aspect of the invention the mixture has an initial melting
temperature of about 232.degree. C. and a remelt temperature in a
range from about 240 to about 245.degree. C. The Sn powder may have
a powder size is in a range from about 200 mesh to about 325 mesh
and the Sb powder has a powder size in a range from about 200 mesh
to about 600 mesh.
The present invention provides a composition for soldering,
comprising: Bi powder mixed with antimony (Sb) powder, and wherein
a bulk composition of antimony in the mixture is about 10 wt %
Sb.
In this aspect of the invention the Bi powder may have a powder
size in a range from about 200 mesh to about 325 mesh and the Sb
powder has a powder size in a range from about 200 mesh to about
600 mesh, and the mixture may have an initial melting temperature
of about 272.degree. C. and a remelt temperature of about
285.degree. C.
The present invention provides a composition for brazing,
comprising: substantially pure Ni mixed with substantially pure Cu
powder such that a bulk composition of Ni in the mixture is in a
range from about 15 wt % to about 85 wt % Ni.
In this aspect of the invention the bulk composition of Ni in the
mixture is about 25 wt % Ni, and the Cu and Ni powder size is in a
range from about 200 mesh to about 325 mesh, and the composition
has an initial melting temperature of about 1085.degree. C. and a
remelt temperature of about 1175.degree. C.
In another aspect of the invention there is provided a composition
for brazing, comprising: Cu--Ag eutectic alloy powder mixed with Cu
powder such that a bulk composition of Ag in the mixture is between
about 5 to about 8 wt % Ag.
In another aspect of the invention there is provided a composition
for brazing, comprising: Al--Zn eutectic alloy powder mixed with Al
powder such that a bulk composition of Zn in the mixture is between
about 10 to 20 wt % Zn.
In another aspect of the invention there is provided a composition
for brazing, comprising: Cu--P eutectic alloy powder mixed with Cu
powder such that a bulk composition of P in the mixture is between
about 1 to 2 wt % P.
In another aspect of the invention there is provided a composition
for brazing, comprising: Cu--Mn alloy powder mixed with a Ni powder
such that the composition of the braze is about 80% Ni, 14% Cu and
6% Mn.
In another aspect of the invention there is provided a composition
for brazing, comprising: Cu--Sn alloy powder mixed with a Ni powder
such that the composition of the braze is 80% Ni, 10% Cu, and 10%
Sn.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described, by way of example
only, reference being had to the accompanying drawings, in
which:
FIG. 1 is a plot of temperature difference versus temperature of an
exemplary variable melting point solder produced in accordance with
the present invention taken from a differential thermal analyser
(DTA);
FIG. 2 shows the results of differential scanning calorimetry for a
large Pb/large Pb--Sn eutectic powder combination;
FIG. 3 shows the results of differential scanning calorimetry (DSC)
for a small Pb/small Pb--Sn eutectic powder combination;
FIG. 4 shows the results of differential scanning calorimetry for a
solder paste containing 90 wt % of small Sn and 10 wt % of large Sb
particles;
FIG. 5 shows the results of differential scanning calorimetry for a
solder paste containing 90 wt % of large Sn and 10 wt % of large Sb
particles;
FIG. 6 shows the results of differential scanning calorimetry for a
solder paste containing 90 wt % Bi and 10 wt % Sb powder;
FIG. 7 shows is a DSC trace showing the melting/remelting behavior
of a braze of Cu-25 wt % Ni made using the medium Ni particles;
and
FIG. 8 is a diagrammatic representation of the different stages of
melting of a variable melting point solder paste produced in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A solder or braze composition produced in accordance with the
present invention is made by mixing a low melting point pure metal
or alloy powder together with a higher melting point pure metal
powder. As used herein, the term pure or substantially pure means a
metal having a composition equal to or exceeding 98% of the metal
with other elements different than the elements in the mixture not
to exceed 0.5%. (i.e. in the Pb--Sn system described hereinafter,
substantially pure Pb metal is 98% Pb or greater, it can have up to
2%Sn in it but may not have anymore than 0.5 wt % of any other
alloying element. In most cases it would be "commercially" pure
with a composition of 99.5% Pb or better.)
The low and high temperature metal or alloy powders are chosen such
that they have considerable mutual solid solubility so that in-situ
alloying occurs between the low and high melting point powders such
that solidification occurs with no or little intermetallic
formation compounds being formed between them during the in-situ
alloying process. The formation of significant intermetallic
compounds within the solder structure can be detrimental to the
properties of the resulting solder joint. The bulk composition of
the mixture is chosen such that its solidus temperature is equal
to, or above, the soldering temperature,
The objective of forming a solid solder or braze joint at the
processing temperature requires the amount of initial liquid
forming component be controlled which, in turn requires a bulk
solder or braze composition with a higher amount of high melting
point phase than the prior art solder compositions (generally
greater than 90 wt %). It is important that the bulk composition
must be chosen such that its solidus temperature is equal to or
higher than the soldering process.
This mixture may be further mixed with a flux to form a paste or
compacted (or tape cast) to form preform wires and/or sheets. The
flux used to form the paste can be chosen from compositions
commonly used in the art of making conventional solder pastes.
Suitable formulations include rosin dissolved in isopropanol as
well as activated rosin solutions which included 10 to 20 wt %
additions of an organic acid solution such as DMA-HCL.
All powder sources have a distribution of particle sizes, where
particle size is defined as the maximum diameter of an
approximately spherical particle. The "mean particle size" referred
to in this patent refers to a measurement of the average particle
diameter of a mixture, or in the case of a mesh size range, to a
minimum and maximum particle diameter of the powders.
The paste or preform is placed between two components to be joined.
The assembly is heated to a maximum temperature, which is below the
melting point of the higher melting point powder. During heating
the low melting point powder melts and spreads throughout the paste
or preform consolidating the powder mixture and joining the two
components together. With further heating the liquid metal and high
temperature solid metal powders undergo in-situ alloying. This
alloying process transforms the liquid to a solid phase and thus
the solder freezes or solidifies at the maximum temperature of the
soldering process. For this to occur the soldering temperature must
be equal to or less than the solidus temperature of the bulk
composition of the solder powder mixture. This process occurs to
the extent that no (or very little) liquid metal remains to
solidify during the cool down process and a solid joint is formed
at the solder temperature. The absence of a liquid phase during
cool down avoids the formation of low temperature phases, which can
segregate to particle or grain boundaries and be detrimental to
physical and mechanical properties. Upon reheating, the solder or
braze will not remelt until a temperature greater than the previous
maximum temperature is reached.
During the process, enough liquid phase must initially form to
consolidate the paste or preform and spread over the base materials
to form the joint. However a limit is placed on the amount of
liquid present in order that it may be solidified at the maximum
temperature to form a solid joint. The length of time over which
the liquid is present it also important. If the liquid
re-solidifies too fast it may not have time to cause consolidation
and joining. Conversely it may be present so long that it never
solidifies at the solder or braze temperature. Controlling these
features of the liquid phase requires careful selection of the
powder sizes.
The powder size ranges of the different constituents are chosen
such that the maximum amount of initial melting occurs, (i.e.
in-situ alloying prior to melting is minimized) and that the rate
at which this liquid resolidifies at the solder or braze
temperature is also maximized (i.e. the rate of in-situ alloying
after melting is maximized). The absolute powder sizes for a
particular case will depend on the individual powder combinations
chosen and the intended application.
More particularly, the absolute diameter of the particles is
important as well as the relative diameters of the low and high
melting point powders. The exact nature of the particle choices is
dependent on the systems under consideration. For example, in
systems where the low melting point liquid solidifies too slowly,
both low and high melting point powder should have small diameters
(e.g. 500 to 600 mesh) to promote solidification. In systems where
the low melting point liquid solidifies too fast the low and high
melting point powders should have large diameters (e.g. 200 to 325
mesh) to delay solidification. Intermediate rates of solidification
can be achieved by choosing different particle diameters for the
low and high melting point powders (i.e. large high and small low
melting point powders or small high and large low melting point
powders).
A major advantage of the present composition is that by using two
alloying components with a larger content of high melting point
phase, and the fact that solidification occurs at the soldering
temperature, the formation of detrimental low melting point phases
which segregate to particle or grain boundaries is reduced or
avoided.
The present invention will now be illustrated by the following
exemplary and non-limiting examples.
EXAMPLE 1
A solder paste having a maximum solder temperature of 220.degree.
C. was chosen. In this case a Sn--Bi alloy was used. Pure Sn powder
was mixed with a Sn--Bi eutectic powder with a composition of 43 wt
% Sn:57 wt % Bi such that the bulk composition of the mixture was 3
wt % Bi. This composition was chosen because its solidus
temperature is about 220.degree. C. The powders were mixed with
paste chemicals common in the art. More specifically a paste
containing 90 wt % of the powder mixture and 10 wt % of an
activated resin solution was found to be a suitable
formulation.
The paste was heated to 220.degree. C. in a Differential Thermal
Analyser (DTA), which allows the measurement of melting events. The
sample was then cooled to 75.degree. C. and then reheated to
220.degree. C. in the same instrument to measure its re-melting
characteristics. The trace from the DTA is shown in FIG. 1. As FIG.
1 indicates, the first time the paste is heated, the low melting
point Sn--Bi powder melts at 140.degree. C. This is followed by
in-situ alloying between the Sn powder and the liquid, which leads
to some secondary melting. The absence of a solidification peak
when the sample is cooled indicates that the sample substantially
froze at the solder temperature. When the sample is reheated, no
melting takes place until a temperature in excess of 220.degree. C.
(more specifically 225.degree. C.). Metallographic examination of
solder pastes heated to 220.degree. C. indicates that coalescence
of the solder occurs such that a density of greater than 95% is
achieved. In addition, no low melting point eutectic phases were
found in the processed solders.
For this system, the Bi composition could range from about 2 to 19
wt % depending on the application. The initial melting temperature
will remain fixed as the Bi content changes, however the remelt
temperature will not. The remelt will range from about 225.degree.
C. at Bi=2% down to close to 140.degree. C. at a Bi content of 19%.
R=22.degree. C. Therefore the choice of composition determines the
amount of melting point shift and the peak solder or braze process
temperature that can be used (the maximum process temperature
should not exceed the remelt temperature of the mixture),
EXAMPLE 2
A solder paste having a maximum solder temperature of 250.degree.
C. was chosen. In this case a Pb--Sn alloy was used. Pure Pb powder
was mixed with a Pb--Sn eutectic powder with a composition of 62 wt
% Sn:38 wt % Pb such that the bulk composition of the mixture was
15 wt % Sn. This composition was chosen because its solidus
temperature is about 250.degree. C. The powders were mixed with
paste chemicals common in the art. More specifically a paste
containing 90 wt % of the powder mixture and 10 wt % of an
activated rosin solution was found to be a suitable
formulation.
To illustrate the influence of powder size over the
re-solidification of the liquid phase, several powder size
combinations were tested. In the following the term "large
particles" refers to powder in the size range of 200-325 mesh and
"small particles" refers to powders in the size range of 500-600
mesh. A large diameter of the high melting point powder (i.e. Pb)
was mixed with either large diameter particles or small diameter
particles of the Pb--Sn eutectic low melting powder, thus creating
a first and second powder size combination. A small diameter of the
high melting point Pb particles was mixed with either large
diameter particles or small diameter particles of the Pb--Sn
eutectic low melting powder, thus creating a third and fourth
powder size combination. A paste of each of the four powder
combinations was heated to 250.degree. C. in a DSC. The samples
were then cooled to 115.degree. C. and then reheated to over
300.degree. C. in the same instrument to measure its re-melting
characteristics.
The trace from the DSC for a large Pb/large Pb--Sn eutectic powder
combination and small Pb/small Pb--Sn eutectic powder combination
are shown in FIGS. 2 and 3 respectively. Both solder pastes have
similar initial melting characteristics. The first time the pastes
are heated, the low melting point Pb--Sn powder melts at
183.degree. C. No additional melting takes place up to the peak
temperature of 250.degree. C. While not evident in the 1.sup.st
heating cycle, in-situ alloying does occur between the eutectic
liquid and the Pb particles. In-situ alloying is indicated when the
samples are cooled and then reheated. For the case of the small
Pb/small Pb--Sn eutectic powder combination, no solidification of
eutectic occurs during cooling and very little remelts upon
reheating. This indicates the eutectic liquid did solidify at the
peak temperature by in-situ alloying.
Upon reheating melting begins at a temperature of about 240.degree.
C. and continues to a temperature of 290.degree. C. Pure Pb has a
melting point of 327.degree. C. Therefore significant in-situ
alloying between the Pb and eutectic powder during initial heating
is the only way in which a melting range between 240-290.degree. C.
could have occurred upon reheating. The results of FIG. 3
demonstrate that an increased melting point of over 50.degree. C.
can be achieved in this solder paste system.
The large Pb/large Pb--Sn eutectic powder combination does indicate
solidification of eutectic liquid during cooling and some eutectic
remelt during heating. This amount of liquid represents less than
25% of the original liquid present, so significant solidification
of the liquid phase did occur at the peak temperature. The presence
of a melting range between 240-290.degree. C. upon reheating also
indicates that significant in-situ alloying occurred.
However the use of this powder size combination has slowed down the
solidification of the liquid during soldering compared to the
small/small powder combination. Table 1 gives results for the
amount of eutectic left upon re-heating for all four powder size
combinations as determined by the type of DSC experiments described
in FIGS. 2 and 3. Clearly the choice of powder size combination
determines the rate (or amount) of low melting point phase that can
be re-solidified during soldering. Large/large combinations would
be chosen if the consolidation and joint formation required the
liquid to be present longer. Holding the solder joint at peak
temperature longer than that done in the (differential scanning
calorimeter (DSC) experiments would eventually solidify the solder
even in the large/large case so that its remelt characteristics
would be similar to that of the fine/fine case and a melting point
shift of 50.degree. C. could be achieved. In this system the Sn
content could range from 5 to 21 wt % The same reasoning in respect
of the remelt temperature discussed above with respect to Example 1
applies in this example also. The remelt temperature will range
from about 300.degree. C. (5%) down to 183.degree. C. (21%).
While this example was based on a Pb--Sn system similar results
would be expected for the Sn--Bi alloy described in the first
example.
EXAMPLE 3
A solder paste having a maximum solder temperature of 240.degree.
C. was chosen. In this case a Sn--Sb alloy was used. Pure Sn powder
was mixed with pure Sb powder such that the bulk composition of the
mixture was 10 wt % Sb. This composition was chosen because its
solidus temperature is above 240.degree. C. The powders were mixed
with paste chemicals common in the art. More specifically a paste
containing 90 wt % of the powder mixture and 10 wt % of an
activated rosin solution was found to be a suitable formulation.
The paste was heated to 240.degree. C. in a DSC. The sample was
then cooled to 200.degree. C. and then reheated to above
290.degree. C. in the same instrument to measure its re-melting
characteristics.
The influence of powder size was also investigated in this system
to illustrate a different powder size effect than that seen for a
system like Pb--Sn described in example 2. In the first case "small
Sn particles" (i.e. 500-600 mesh) were mixed with "large Sb
particles" (i.e. 325 mesh). The trace from the DSC is shown in FIG.
4. The first time the paste is heated, melting begins at a
temperature of 232.degree. C. (i.e. the melting point of pure Sn).
More melting takes place when heating up to 240.degree. C. When the
sample is cooled, some solidification takes place. When the sample
is re-heated very little melting occurs below 240.degree.C.
indicating a successful melting point shift. However both the
initial melting and solidification peaks are relatively small and
much lower than that expected for the amount of Sn present in the
sample (i.e. assuming all the Sn powder melted in its pure
form),
Extensive metallographic examination revealed that significant
interdiffusion between the pure Sn and Sb particles occurs prior to
melting. This reduces the amount of liquid phase produced at lower
temperatures (i.e. between 232-240.degree. C.) reducing solder
consolidation and the extent of the melting point shift. In essence
the in-situ alloying that is counted on to cause a melting point
shift begins in the solid state which is detrimental to the over
all objective of the invention.
To counteract solid state interdiffusion prior to melting large Sn
particles (i.e. 100-200 mesh) were mixed with the large Sb
particles of the previous case. The same heating cycle as above was
used. The DSC trace from this solder paste is illustrated in FIG.
5. The original melting and solidification peaks are much larger
compared to the case of small Sn particles. This indicates that the
use of large Sn particles inhibits solid state interdiffusion
generating more liquid phase at lower temperatures. The re-melt
behavior of the large Sn-large Sb mixture is similar to that of the
small Sn-large Sb mixture. Large/large powder combinations reduce
the contact area between the two powder types thus reducing solid
state interdiffusion.
Clearly in solder combinations where significant solid state
interdiffusion can take place, large/large combinations of powders
should be chosen to maximize the amount of low melting point liquid
phase.
Particle size effects have a different influence over behavior in
this system than seen in the Sn--Bi and Pb--Sn of examples 1 and 2.
This is partly due to the fact that solid state interdiffusion is
not significant in the Pb--Sn and Sn--Bi systems. However examples
2 and 3 point out two important controlling aspects of the variable
melting point process. Both the amount of the initial low melting
point liquid phase and its duration in the powder mixture, are
important to the success of the VMP technology. The choice of
particle size can strongly influence both of these processes and
therefore is an important controlling parameter.
In the experiments of FIGS. 4 and 5 the solder was not held for any
time at 240.degree. C. If a holding period were included, some
isothermal solidification would have occurred limiting the amount
of freezing during cooling. This would lead to a re-melt
temperature a few degrees higher than that shown in FIGS. 4 and
5.
EXAMPLE 4
A solder paste having a maximum solder temperature of 285.degree.
C. was chosen. In this case a Bi--Sb alloy was used. Pure Bi powder
was mixed with pure Sb powder such that the bulk composition of the
mixture was 10 wt % Sb. This composition was chosen because its
solidus temperature is approximately 285.degree. C. A paste
containing 90 wt % of the powder mixture and 10 wt % of an
activated rosin solution was found to be a suitable formulation.
The paste was heated to 285.degree. C. and held at that temperature
for 5 minutes in a DSC, which allows a quantitative measurement of
melting events. The sample was then cooled to 210.degree. C. and
then reheated to 350.degree. C. in the same instrument to measure
its re-melting characteristics. The trace from the DSC is shown in
FIG. 6.
Melting of the powders during initial heating began at the melting
point of Pure Bi (i.e. 272.degree. C.). When the sample cools, only
a small solidification peak is evident indicating that in-situ
alloying has occurred between the Bi liquid and solid Sb such that
the liquid solidified at the solder temperature. Upon reheating the
solder does not melt until the original peak temperature is
exceeded. The compositions exemplified in Example 4 are isomorphous
and the compositional range could be very large depending on the
application. Efficacious solder compositions may be formulated
having between 10 to 80 wt % Bi in the Sn--Bi system,
EXAMPLE 5
A brazing compound having a maximum brazing temperature of
1110.degree. C. was chosen. In this case a Cu--Ni alloy was used.
Pure Ni powder was mixed with pure Cu powder such that the bulk
composition of the mixture was 25 wt % Ni. This composition was
chosen because its solidus temperature (i.e. 1175.degree. C.), is
above the brazing temperature of 1110.degree. C. The brazing
compound was heated in a nitrogen atmosphere at a rate of
20.degree. C. per minute to 1110.degree. C. in a DSC, which allows
a quantitative measurement of melting events. The sample was then
cooled to 925.degree. C. and then reheated to 1110.degree. C. In
the same instrument to measure its re-melting characteristics.
The influence of powder size was also investigated in this system,
in all cases the Cu powder size was fixed at a mesh size of 325.
However this Cu powder was mixed with one of three different Ni
powder sizes. These sizes included "large Ni particles" (i.e.
48-150 mesh), medium Ni particles (i.e. -100+325 mesh) and small Ni
particles (3-7 microns). An example of the melting/remelting
behavior of these brazes is shown in FIG. 7 for the case of Cu-25
wt % Ni made using the medium Ni particles.
Melting of the Cu powder during initial heating began at the
melting point of pure Cu (i.e. about 1085.degree. C.) and the DSC
trace indicates a large melting peak between 1085 and 1110.degree.
C. When the sample was cooled, a large solidification peak is also
evident. However, upon reheating, the braze clearly indicates a
higher melting point onset of about 1100.degree. C. and very little
total melting up to the peak temperature of 1110.degree. C. Cooling
of the braze for the second time also indicates very little
re-solidificatlon. These features on the DSC curve indicate that
significant in-situ alloying has occurred in the powders resulting
in an increased melting point.
Several quantitative measurements can be made from a DSC trace such
as that of FIG. 7. The enthalpy of melting (in Joules/gram (J/g))
of the initial Cu liquid can be measured as well as the enthalpy of
the re-melt peak upon reheating the braze. The magnitude of these
values directly indicate the amount of liquid formed in the braze
compound. These measurements were obtained from a series of
experiments designed to illustrate the influence of Ni particle
size, hold time near the braze temperature and heating rate on the
melt/remelt behavior of a Cu--Ni variable melting point braze
compound. Tables 2 and 3 summarize the results of this work.
In Table 2 the influence of heating rate and Ni powder size on the
initial melt and re-melt peaks are illustrated. At a given heating
rate, the amount of liquid, which forms upon the first heat cycle,
is strongly influenced by the Ni powder size. As the Ni powder size
is reduced, initial liquid formation is reduced. This occurs to
such an extent that, when a small Ni powder size is used, no
initial melting is detected up to the braze temperature of
1110.degree. C. (except at a heat rate of 40.degree. C./min, where
a very small amount of melting is detected). Metallographic
analysis has confirmed that this is due to extensive solid-state
interdiffusion which occurs between the pure Cu and Ni powders.
This interdiffusion prealloys the pure Cu powder to the extent that
its melting point shifts above 1110.degree. C. Table 2 indicates
that, as the sample is heated faster, less time is given for
prealloying, resulting in more initial melting. This is most
evident in the mixtures where large and medium Ni powder was used
in the braze compound. With the use of large Ni powder and a
heating rate of 40.degree. C./min, liquid formation during initial
heating is near 100%. This indicates that prealloying prior to
melting can be avoided by the use of large Ni powders and high
heating rates.
Table 2 also indicates the degree to which the melting point shifts
by measuring the enthalpy of remelting when the sample is reheated
to the braze temperature of 1110.degree. C. As the Ni particle size
is reduced from large to medium the extent of the melting point
shift increases. (A melting point shift has little meaning for the
Fine Ni samples where no initial melting took place).
Metallographic analysis indicates that good consolidation was
achieved in both the Cu-25 wt % large Ni and Cu-25 wt % medium Ni
braze samples. Therefore, since a better melting point shift was
achieved with medium Ni particles, these would be the best
candidate for a variable melting point Cu--Ni braze.
The compositions exemplified in Example 5 are isomorphous and the
compositional range could be very large depending on the
application. Efficacious brazing compositions may be formulated
having between about 20 to 80 wt % Ni in the Cu--Ni system.
The behavior of the variable melting point solders and brazes and
the influence of particle size can be understood and described
using the schematic model presented in FIG. 8. The example in the
figure is based on the Sn--Bi solder of the first Example I but the
principles described apply equally well to Examples 2, 3, 4 and
5.
Depicted in FIG. 8 is an arrangement of pure Sn and eutectic Sn--Bi
powders as they would exist in side the solder paste. In areas
where the Sn and eutectic particles are in direct contact,
solid-state interdiffusion can occur as the solder is heated (i.e.
stage I). In this alloy system interdiffusion primarily results in
the absorption of Bi into the pure Sn particles to form a Sn rich
solid solution near the surface of the particle. This could
actually result in a small reduction in the amount of eutectic in
the material prior to melting. (The Sn--Sb system of example 3 and
the Cu--Ni of example 5 clearly illustrate cases where
interdiffusion in stage I can significantly influence the process).
When the eutectic temperature is reached, the eutectic particles
melt and begin to spread throughout the powder compact by capillary
forces (i.e. stage II). As the liquid spreads it comes in contact
with a large surface area of pure Sn solid. In order to maintain
equilibrium, surface diffusion of Bi into the Sn occurs rapidly,
resulting in in-situ alloying and re-solidification of the liquid
begins (Stage III). During stage II consolidation of the solder and
joining to the base components must occur. Therefore the transition
from stage II to stage III is a critical step. Since in-situ
alloying and re-solidification primarily occurs by surface
diffusion it is very sensitive to the surface area of the pure Sn
(or higher melting point) powder. A high surface area is present
when the solder paste is made with small diameter Sn particles and
therefore re-solidification is rapid. The opposite occurs when the
paste is made with large Sn particles. The size of the low melting
point particles also effects stage III. If the eutectic particles
are small the low melting point phase is inherently uniformly
distributed throughout the powder compact. When this phase melts it
does not need to spread as far to come in contact with solid Sn
surface. Therefore re-solidification can occur more quickly. When
the eutectic particles are large the low melting point phase is
inherently poorly distributed. Therefore when the particles melt
the liquid must flow over longer distances to come in contact with
solid Sn surface. Consequently in-situ alloying and
resolidification occurs more slowly. On this basis combinations
where both particles are large should lead to the slowest
resolidification whereas combinations where both particles are
small should lead to the fastest solidifications. The example 2
clearly demonstrates that this is the case.
In solder or braze formulations where a limited amount of low
melting point powder is present such as in the Sn--Bi and Pb--Sn
solders of Example 1 and 2, slow re-solidification and the use of
large powder combinations will likely achieve the best results. In
formulations where high amounts of low melting powders are present,
such as Sn--Sb, Cu--Ni and Bi--Sb, more rapid solidification may be
required and the use of small particle combinations desired. This
is evident in the Cu--Ni system where medium Ni particles are
better than large Ni particles. However in systems like this where
extensive solid-state interdiffusion is also present, the use of
very small powders may reduce initial liquid formation (e.g. Cu-25
wt % small Ni). In solders or brazes of this type, where a large
amount of initial liquid is present but solid state interdiffusion
is large, an optimum mixture between large and small particles will
give the best results (e.g. Cu-25 wt % medium Ni of example 5).
In solders such as the Sn--Bi alloys stage III is followed by Stage
IV where some of the re-solidified liquid re-melts as the
temperature increases. This leads to secondary melting and further
in-situ alloying followed by re-solidification. In solders like the
Pb--Sn paste stage IV does not occur because the in-situ alloying
produces compositions whose melting point is higher than the solder
temperature (i.e. 250.degree. C. in FIGS. 3 and 4).
Examples 1-5 were chosen because they represent different binary
alloy systems, all of which are suitable for the development of
variable melting point solders or brazes. The Bi--Sb and Cu--Ni
system, with isomorphous phases diagram, exhibit unlimited solid
and liquid solubility. In systems like these the pure metal with
the lowest melting point is chosen as the low melting point powder
addition (e.g. Bi or Cu) and the pure metal with the highest
melting point chosen as the high melting point powder addition
(e.g. Sb or Ni). These systems can generally contain large amounts
of initial low melting point phase such as the Bi-10 wt % Sb paste
of Example 4 or the Cu-25 wt % Ni braze of example 5.
The Pb--Sn and Sn--Bi systems have binary eutectic phase diagrams
and therefore limited solid solubility and the formation of a low
melting point eutectic phase when the solubility limit is reached.
In these systems the low melting point powder is chosen to be at
the eutectic composition and the high melting point powder chosen
from either of the two higher melting point pure powders. In
systems like these the amount of liquid phase is lower than in
isomorphous systems because the bulk composition of the mixture
must be within the solubility limit of the phases.
The Sn--Sb system has a more complex phase diagram that has limited
solubility and the formation of several intermediate or
intermetallic compounds. However in the Sn rich region of the phase
diagram there is some solid solubility of Sb in Sn. Therefore
mixtures of Sn with Sb up to 10 wt % can be used to form variable
melting point solders without forming significant amounts of
intermetallic phase.
The above examples and explanations lay out the methods for
developing and optimizing variable melting point solders and
brazes. Table 3 lists some suggested additional braze compositions
and their initial and re-melt temperatures. The first 3 examples
are binary eutectic systems similar to Pb--Sn and Sn--Bi. Therefore
in the Cu--Ag case the mixture would consist of a 1st powder with a
eutectic alloy bulk composition of 28% Cu:72% Ag and a 2.sup.nd
powder of substantially pure Cu composition, where both powders are
mixed in a ratio such as to obtain a bulk composition of 5 to 8 wt
% Ag. The Al--Zn braze would consist of a 1st powder with a
eutectic alloy bulk composition of 94% Zn:6% Al and a 2.sup.nd
powder of substantially pure Al composition, where both powders are
mixed in a ratio such as to obtain a bulk composition of 10 to 20
wt % Zn. The Cu--P braze would consist of a 1st powder with a
eutectic alloy bulk composition of 8.3% P:91.7% Cu and a 2.sup.nd
powder of substantially pure Cu composition, where both powders are
mixed in a ratio such as to obtain a bulk composition of 1 to 2 wt
% P.
The 4th example is a ternary isomorphous phase diagrams system
similar to Bi--Sb or Cu--Ni. It would consist a 1st powder with a
alloy bulk composition of 70% Cu:30% Mn and a 2.sup.nd powder of
substantially pure Ni composition, where both powders are mixed in
a ratio such as to obtain a bulk composition illustrated in Table
3. The 5.sup.th example is a ternary alloy phase diagram system
similar to Sn--Sb. It would consist a 1st powder with a alloy bulk
composition of 50% Cu:50% Sn and a 2.sup.nd powder of substantially
pure Ni composition, where both powders are mixed in a ratio such
as to obtain a bulk composition illustrated in Table 3. The primary
difference between the systems of Table 3 and the systems of
examples 1-5 is that their compositions and melting temperatures
differ. Many of the principles of variable melting point behavior
described in this invention will also apply to the brazes of Table
3.
The foregoing description of the preferred embodiments of the
invention has been presented to illustrate the principles of the
invention and not to limit the invention to the particular
embodiment illustrated. It is intended that the scope of the
invention be defined by all of the embodiments encompassed within
the following claims and their equivalents.
TABLE 1 Re-melt characteristics of Pb-15% Sn variable melting point
solders with different powder size combinations. % re- Powder
combination solidified Small E/small Pb 98% large E/small Pb 90%
Small E/large Pb 80% Large E/large Pb 77%
TABLE 2 Quantitative measurements of enthalpy of melting and
remelting, made from DSC curves, as a function of heating rate and
Ni powder size. 10 20 40 Sample (.degree. C./min) (.degree. C./min)
(.degree. C./min) Heating Melt Re-melt Melt Re-melt Melt Re-melt
rate peak* peak** peak* peak** peak* peak** Cu-25 wt % large Ni 86
12 92 11 99 11 Cu-25 wt % medium 69 3.1 70 3.6 78 4.3 Ni Cu-25 wt %
small Ni 0 0 0 0 1.6 0 *This value of the enthalpy of melting is
expressed as a percentage of that expected if all of the Cu in the
mixture melted in its pure form. **This value of enthalpy of
remelting is expressed as a percentage of the enthalpy measured
from the original melting peak.
TABLE 3 Brazing compositions with low and high temperature variable
melting point brazing compositions Initial melt Re-melt Alloy
Temperature temperature system Composition (.degree. C.) (.degree.
C.) Cu--Ag 5 to 8% Ag 780 950 Al--Zn 10 to 20% Zn 380 500-600 Cu--P
1 to 2% P 714 900-950 Ni--Cu--Mn 20% Cu&Mn, 871 >1200 bal.
Ni Ni--Cu--Sn 20% Cu&Sn, 600 >1100 bal. Ni
* * * * *